1. Field of the Invention
This invention relates generally to wrapping or packaging machines, and more particularly to a device for controlling the sealing of thermoplastic overwraps, i.e., to overwrapping machines whose task is to wrap in sheets of transparent plastic, products such as cigarettes previously packed in wraps that are essentially parallelepiped in shape.
2. Description of Related Prior Art
Conventionally, cigarette packages are wrapped and sealed in sheets of transparent plastic material such as "cellophane" or polypropylene on overwrapping machines. The plastic materials are heat-sealable by heating elements which are cam-actuated to engage the cigarette package along the side and ends thereof.
The process of wrapping cigarette packages with polypropylene film has been automated by several types of machinery over the years. The most dominant and successful of all designs is the G. D 4350. This is an Italian machine of particularly good quality and design. The G. D 4350 machine performs its function smoothly and reliably. The operation of this machine is described in a series of patents issued to Enzo Seragnoli including U.S. Pat. No. 3,967,767 issued Jul. 6, 1976, U.S. Pat. No. 3,979,881 issued Sep. 14, 1976, U.S. Pat. No. 3,982,380 issued Sep. 28, 1976, U.S. Pat. No. 3,984,963 issued Oct. 12, 1976 and U.S. Pat. No. 4,134,502 issued Jan. 16, 1979. Other patents dealing with the process of wrapping cigarette packages with plastic film include U.S. Pat. No. 4,330,977 issued to Heinz Fock on May 25, 1982, and U.S. Pat. No. 4,585,503 issued to Nils Von Wickert and Jurgen Steinhauer on Apr. 29, 1986. There are, however, shortcomings in the heat seal area that become more obvious under close inspection.
The action of sealing polypropylene film is controlled by three factors. These factors are 1) contact pressure, 2) heat, and 3) contact time. The contact pressure is fixed by the setting and alignment of the heaters themselves combined with the in/out stroke generated by a cam lift. The heat should remain relatively constant through the control loop created by a thermocoupling and thermostatic voltage controller. That leaves the third factor of contact time.
As the machine increases or decreases in RPM speed, the time value for each revolution will fluctuate as well. This fluctuation will vary the actual clock time value that the heaters touch the film. As a direct result, the increase in clock time for contact that occurs at lower speeds will burn the film. The working temperature for heat-sealing polypropylene is approximately in the range of 125-165 degrees C. Because it is impossible to rapidly change the heat value or contact pressure the only factor left to vary is, of course, the contact time.
The G. D 4350 machine makes an attempt at regulating the contact time to the lower RPM values by having a dual-faced cam design. One face of this cam set is configured to operate at the higher machine function speeds (about 360-400 RPM). At these higher speeds, the contact time has been set at about 1/10th of one second. At this value, the heat range for the heaters can be set at about 140 degrees C. Of course, the factor of heat has a negative effect on the moisture content of the cigarettes. As the machine slows to below 180 RPM the cam faces shift to allow a shorter contact time. This reduced contact time value allows the heater face to contact the cigarette package for a shorter clock time. This clock time, however, has proven to be incorrect for most low-speed values.
Since the G. D 4350 cam face design allows for only two distinct contact time dwell values, it is not adequate to the task. My improvement provides for a coordinated contact time value more precisely matched with the RPM of the machine over its entire operating range.